Photographer: Robert Arn
The photo above featuring a classic lunar halo was taken from Friends Creek Park in Illinois. The Moon was full this first night of December 2009, and though 22 degree halos can be observed when the Moon is in the gibbous phase, they’re more obvious when the Moon is at its brightest. Like their counterpart during the daytime hours (solar halo), lunar halos form when moonlight interacts with pencil shaped ice crystals in cirrus clouds. These crystals have no special orientation, and therefore the refracted moonlight forms a circle around the Moon. As moonlight passes through the side faces of these crystals its refracted 22 degrees from the initial angle of incidence angle before exiting through alternate side face crystals. Also note the band of light, corona, immediately outside the Moon.
Summary Author: Robert Arn; Jim Foster
The Moon this night was in the constellation of Taurus the Bull. Aldebaran, the brightest star is Taurus, is just below and to the right of the Moon. Look closely and you can find Orion; tangent to the halo at lower right. Gemini is also visible to the lower left of the halo, as well as the Pleiades star cluster which appears within the halo just above, and to the right of, the Moon.
Photo details: Canon XSi camera(unmodified); Sigma 10 mm f/2.8 fisheye lens; Still tripod mount; 1 x 5 seconds; ISO 200; f/3.2 (for clouds, stars, and halo); 1 x 3200 seconds; ISO 200 f/3.2 (for Moon detail); processing using Digital Photo Professional, Photoshop.